Subtract.
There are many ways to solve this problem. Let's see two ways we can solve it. Place Value We can think in terms of tenths: $\begin{aligned} &\phantom{=}42.4 - 31\\\\ &=42.4-31.0\\\\ &=424\text{ tenths} - 310\text{ tenths}\\\\ &=114\text{ tenths}\\\\ &=11.4 \end{aligned}$ Place value table Let's line up the numbers by their decimal places. Tens Ones $.$ Tenths $4$ ${2}$ $.$ ${4}$ $3$ $1$ $.$ ${0}$ Now, let's subtract each place value column. Tens Ones $.$ Tenths $4$ ${2}$ $.$ ${4}$ $-$ $3$ ${1}$ $.$ ${0}$ $\underline{~~~~}$ $\underline{~~~~}$ $\underline{~~~~}$ $\underline{~~~~}$ $1$ $1$ $.$ $4$ $11.4 = 42.4 - 31$